Low-temperature vitreous enamels



high temperatures.

Patented Aug. 26, 1952 2,608,490 LOW-TEMPERATURE VITREOUS ENAMELS John W. Donahey, Drexel Hill, Pa., assignor to Foote Mineral Company, Philadelphia, Pa., a corporation of Pennsylvania No Drawing. Application August 1, 1950,

Serial No. 177,117

12 Claims. 1

The present invention relates to a novel composition adapted for use in the preparation of vitreous coatings for metal articles. More particularly, the invention relates to a composition in the form of a frit, adapted for use as a vitreous enamel in the coating of articles of light metals, such as aluminum, aluminum alloys, magnesium, magnesium alloys, and the like.

Aluminum and aluminum alloys, because of their high strength and light weight, have found increasing use in the manufacture of articles for industrial purposes. Aluminum and aluminum alloys, however, are subject to attack by various substances which is not only detrimental from the standpoint of deterioration of the metal article, but also from the standpoint of contamination of substances brought into contact with the metal article. It has been proposed to apply vitreous enamel coatings to the aluminum or aluminum alloy article in an attempt to protect the metal. There are many problems presented in the application of vitreous coatings on aluminum and aluminum alloys which make most of the presently known vitreous compositions Wholly unsuitable as protective and decorative coatings therefor. In the first place, aluminum has a very high coefficient of thermal expansion. The expansion of articles made of aluminum or of aluminum alloys at elevated temperatures thus results in the destruction of vitreous coatings.

available enamel compositions have coefficients of thermal expansion much less than that of aluminum and thus are unsuitable, from this standpoint, for use on metal articles made from aluminum or aluminum alloys.

Moreover, ordinary vitreous enamels, particularly those high in P205, contain small quantitles of water (or hydrogen ion) within the glass structure itself. Because of the reducing action of aluminum at elevated temperatures, hydrogen, as a result, is evolved at the metal-enamel interface as a gas. These gas bubbles destroy adherence of the enamel to the metal. In addition, most existing vitreous compositions which might be suitable for coating aluminum and aluminum alloys from the standpoint of chemical resistivity and water-insolubility, must be fired at relatively Pure aluminum melts at about 1200 F. and aluminum articles become de formed and lose tensile strength at temperatures well below this temperature. In addition, certain aluminum alloys are deleteriously afiected at temperatures of about ,1000" F. Thus a vitreous enamel having a relatively low maturing temperature at which the metal is not adversely aflected is much to be desired. It has also been article first in an oxidizing bath followed by a pre-firing of the metal article. Some or allof the problems referred to above in connection with aluminum and aluminum alloys may also apply to magnesium and its alloys.

Relatively recently, a composition has been developed primarily as a vitreous coating for light metals including aluminum, with the view of overcoming certain of the above difiiculties. This composition, however, is composed primarily of lead oxide and silica with minor amounts of other oxides. Lead oxide, as is well known, is toxic, and this characteristic constitutes a considerable health hazard in commercial production. An important feature of the composition of the present invention is that it does not require lead oxide as a constituent thereof, and, in fact, is preferably free of lead oxide.

It is an object of the present invention to provide a composition available for use as a vitreous enamel in the coating of light metals, wherein the above-mentioned difliculties are overcome.

Another object is to provide a vitreous enamel composition which will not be destroyed when applied to articles of light metals, such as aluminum, aluminum alloys, magnesium, magnesium alloys, and the like, when the article is subjected to high temperature.

Still another object is to provide a composition available for use as a vitreous coating for light metals which may be fired at temperatures lower than existing coatings for light metals.

Still another object is to provide a composition which when applied as a vitreous coating to articles of light metals, particularly of aluminum or aluminum alloys, does not result in-the formation of gas bubbles at the enamel-metal interface resulting in loss of adherence.

A further object is to provide a composition available for use as a vitreous coating for light metals, such as aluminum, aluminum alloys, magnesium, magnesium alloys, and the like, which permits the simplification of the pre-treatment of the metal article and does away with the cumbersome pre-treatment procedures encountered with The novel composition of thepresent invention,

in the form of a frit, comprises between about 5 and about mol percent oi LiQO, between about 15 and about 33 mol percent of Nazi),- between about 3 and about 12 mol percent of an easily reducible oxide selected from the group consisting of CuO, C00, CdO, and NiO, between about 15 and about 23 mol percent of A1203,

between about and about 32 mol percent of PzO s, from 0 to about 15 mol percent of B203 encl 'fiiiorin'e an amount equivalent to be twee about" 2 and about" 15" mol percent of Namgthe sum of the mo'l's" of 3203* and A1203 divided by the mols of P205 providing a' ratio ofbetweenabout .o'a'rid about 1.6 and'the sum 'ef' hefnbls oi 13203- and of A'lzOo" and of P265 d; ided by V the sum of the" mols; of: LizO and NatO and ofthe' said easily reducible oXi'de'providing a ratio between about 1 and about 1.5.

The above analysis is not intended" to exclude smailyamouhts of other reactible oxides which may be included for various purposes. For in tense, a small amount of K20 may be present the eoinpositioni If K2O is' present, it'will be excess of 5 mol-percent, more generally" not ingezioess or 2 mol percent or the composition. small amount of-thealkaline earth des, ziii'o oxide of antimony trioxida may pi' s"r it.- Of the alkaline earth oxides, B'aO v "orefavorablefrorri the standpoint of favoring fiieibilityi Zinc oxide 'favor's: chemical resistivity. If such oxides are present;- they will not' be in exo'ssor'5' o1 percent; and more generally not ineness (if- 2 mol'peroent; The'preferred comp'os one or the present invention: will be: sub- 'stanti'a lly free or K20 alkaline earth oxides, ZnO, or SbzOz, consisting essentially of the oxides setro'rtn in the foregoing. paragraph. as will be discussed more in: detail smarter,- coxiventionai coloring" and o'pac'ifympounds may be added to the compositioh of the invention in accordance with known practice.

, The above-described composition: has been found to possess many unusual characteristics, making it'- e'specially'useful as a vitreous eiiaiiie'l compositionifor articles made or aluminumor aluminumalloys, In the first piece; the composition in thefo'r'in of a'vitrified coating possesses a coe'flicient of thermal expansion" approaching that: of aluminum. Thus, when articles made of aluminum or of aluminum alloys are sub- .jected to elevated" temperatures, there is no danger" of the coating becoming cracked or crazed or" otherwise: destroyed; In addition, it

been found that With the com osition of the present invention, there is no 'format'ion of gas bubbles at the enamel-metal interface which WOlild 'I's'lilt in the 105$ 0f adherence b tween the enamel coatin and the metal. This is believed to" be dile' to the fact that the easily reducible oxide of the type mentioned is" more easily. i=8Gilli-0561 by the aluminum iii the article than is water; Thus, instead of reducing the -oiiide becomes reduced by the aluminum to the elemental-metal form, and thisiiietal is deposited at an enamel-metal interface inhibiting further water (or hydrogen ion) in the vitreous coatingably low firing temperatures.

4 reaction. It has also been found that with the composition of the present invention, the cumbersome pre -tr'eatment procedureiof the metal article to be coated is not required. Thus, in most cases, all that is needed by way of pretreatment of the metal article is normal removal ofgrease and. other dirt from the surface as by the use ofco'mmonsoaps or detergents, although it is preferred practice also to slightly etch the surface with thehelp of alkaline or acid solutionsi- Another" important feature of the compositions of thepresent invention is that they maybe fired at temperatures as low as 900 F., in other Words, they have maturing temperatures as low as 900 F. As stated previously, the compositions, of the present invention do not require lead: oxide as a: constituent .there of. Thisfreedom" from lead oxide thus elimihates the health hazard encountered previously in c'ases where lead' oxide was used? The .compositionseof the present 'inventionare also characterized by good: mechanicaladherence-to the metal and chemicalv resistiv-ity.- The properties of adherence, resistivity, and lowmaturi-ngtemperature are, ofcourse, somewhat dependent upon the chemical compositions selected; However, the compositions having the preferred analysis as set forth below exhibit excellent mechanical" adherence." I a Referring further to'the composition of the presentinvention, it hasbeen foundthat if the LizO'iS present'ii'ian amount less than about 5 mol percent, the coating is too solublefor most purposes and requires a. relatively high firing temperature; and if: theLizO is present-inan amount higher: thanabout' 15 percent, thecoating must also be fired at relatively high temperatures; With respect to the NazO, if. the amount thereof is outside the above-recited range, the coatinghasbeen found to require relatively high firing temperatures; In order to provide the elimination of gas bubbles at the enamel-metal interface, ithasbeen found that the easily reducible oxide of the typesetforth above must-.bepresent' inan amount of at'least about 3 mol percent. However, if the amount thereof is higher than about 12 mol percent, the composition requires. excessivelyhigh firing temperatures. With respect to the' A1203 and the P205, it has been found that. if these materials are notwithin the rangesset forth v above, the composition is too soluble for most practical purposes or requires excessively high firing temperatures. With respect tothe- B203, it has been foundthat if this material is present in an amount'greater than about 15 mol percent, the coating is too soluble'for'm'ost practical purposes and becomes excessively hygroscopic, and the int tends to hydrate upon water quenching. However, amounts less than about 1 5".mo'l per:- oent'of B203, forexarnple', from about 2 to about 15 mol percent; preferably from about 6 to 12 mol percent, are advantageously present, as the presence of the B203 lowers the firing temperatu're of the composition. The above-described oxide proportions thus provide proper chemical resistivity and" insolubiliiy coupled with reason- The preferred compositions of. the present invention will. comprise between about 8' and about 13' mol percent of Li'zO, between about 25' and about" 30 mol percent of Nai'O, between about 4 and about 6 P205, between aboutzandabout 12 mol percent of B203, 'andxfiuorine in the amount stated above; and willhave-a ratioof the sum of mols of 13203 and A1203 divided by the mols of P205 of between about 0.8 and about 1.2, and a ratio of the sum of mols of B203 and of A1203 and of P205 divided by the sum of mols of H20 and of Na2O and of easily reducible oxide of between about 1.2 and about 1.4.

Referring to the 'easily' reducible oxide mentioned above, it may beselected from-the group consisting of CuO, CoO, NiO, and CdO. Mixture of two or more of these oxides may be employed if desired. All these oxides are di-valent and are more easilyreduced by aluminum at elevated temperatures than is water; of the easily reducible'oxides, CuO, C00, and N10 are preferred from the standpoint of ease of reduction, and of these, CuO is most easily reducible and thus most advantageous for use, with C of second preference. The easily reducible oxide may impart a characteristic color to the resulting enamel coating. Thus CuO will impart a blue-green color to the coating; 000 will impart a blue color, and NiO will impart a brown color, while CdO imparts no color to the coating. For many purposes, these colored effects are not disadvanta geous, however, if difierent colors are desired they may be provided by the use of conventional coloring agents of the type discussed hereinafter.

The composition, in the form of 2., frit, having the above described chemical composition, may :be' prepared in accordance with conventional practice. As is well-known, in preparing the frit, compounds ultimately providing the desired oxide, analysis, in the present case, providing the above-described oxide analysis-are mixed tosodium carbonate monohydrate, and the like,

phosphates, for instance'tetrasodium pyrophosphate, sodium-tripo1yphosphate, sodium hexametaphosphate, and the like, sodium nitrate, bo-

rates, for instance, borax, and the like; the easily reducible oxides may be provided, for example, by carbonates of the corresponding metal, or the free oxides themselves may be used; the A1203 may be provided, for example, by aluminum metaor ortho-phosphate, aluminum hydroxide, or the free aluminum oxide itself may be used; the P205 may be provided, for example, by the various phosphates such as the sodium phosphates, for instance those mentioned above, ammonium phosphates, for instance monoam monium phosphate, dibasic ammonium phosphate, and the like, tribasiclithium phosphate, the aluminum phosphates, for instance those mentioned above, and the like; the B203 may be provided, for example, by borates, such as sodium borates, for instance borax, boric acid, and the like; the fluorine may be provided by any of the alkali or alkaline earth metal fluorides such as sodium fluoride, lithium fluoride, calcium fluoride, barium fluoride, and the like, aluminum *fluoride, and more complex fluorides, for inbe solution colorants.

stance, cryolite, and the like. It will be'understood, of course, that during melting in thepreparation of the frit, a'certain amount of fluorine islost due to volatilization, the exact amount depending upon the fritting procedure followed. The amount of the source of fluorine will, therefore, be selected to provide a fluorine content in the frit as specified above. One compound may provide some or all of a plurality of the abovementioned oxides, for example, sodium phosphate may be relied upon to provide Na2O and P205. In any event, the compounds will be selected in accordance with well-known practice to provide the proper chemical analysis and in the event such compound contains the constituent not desired in the ultimate frit, such constituent will be of a volatile nature so that it may be removed during the heating and melting of the mixture. For example, where carbonates are employed, carbon dioxide is liberated and where ammonium compounds, such as ammonium phosphate, are employed, the ammonia is liberated.

The materials, in fine particle size, providing the desired chemical analysis upon melting thereof, are mixed together in accordance with common practice and heated to an elevated temperature to provide a molten, pourable mass.

With the compositions of the present invention, temperatures in the neighborhood of from about 1800 to about 2000 F. will be required to provide the molten mass. The molten mass is then quickly chilled such as by pouring it into a water bath, and such quick chilling causes the glasske mass to fracture into small pieces. These small pieces are recovered and dried to provide the frit having the chemical analysis as described above.

The resulting frit may be too coarse for direct use as an enameling composition and may, therefore, be ground to the desired size in accordance with Well-known practice. As stated, the composition of the invention may have mixed therewith an opacifying agent and the opacifying agent may, in accordance with known practice, be incorporated with the frit during milling. This insures thorough distribution of the opacifier throughout the composition. Opacifiersare wellknown and are essentially materials which, in relation to the remainder of the composition, are refractory and which thus do not fuse and dissolve in the composition during firing thereof They thus remain suspended and dispersed throughout the coating in the form of fine particles imparting opacity to the coating. Examples of opacifiers are zirconium oxide, titanium oxide, other high melting compounds of zirconium and titanium, and of antimony, and the like.

Coloring agents may also be added to the composition, if desired, in accordance with known practice, and these may also be added to the composition during milling thereof. The coloring agents employed may be inert pigments or may Examples of coloring agents are the various high melting colored cadmium salts, cobalt salts,- and the like.

Other additives of the type normally incorporated in vitreous enameling compositions may be added to the composition of the invention. For example, it is normally the practice to'incorporate a suspending agent in the composition, usually during milling of the frit. A particularly advanele tro e; t. s a s ormally comp lation, S ch alt-noccul tes when Water is; a de thereto. to nror dethe Slipi ndi 11 5. e l-V as r l .On hemscflfiliflf:thfifil n: E mp so o oh salts are: the). water-sclubla. par icul rly ealk li metal, chloridesnitrat s, phosphate an the like. 1

- The-resu t ng finely gr ndLmixt r before n.- plication; as. an, enamel, will have incorporated therewith alliquid vehicle, most enerally water, toimpart fluidaflow.characteristic, to the material, and; thus to; provide the, slurry or slip which is applied to; the metal. articles. In accordance with oommonlpractice, some. or all of the. water may beaddedzatlthemilll Some or all of the above.- described. additives which are water-soluble may be rnixedtwiththe: water, and in thisway incorporated; in the. enamel. For instance, the suspending-agent and/or the electrolyte salt may be added to .the' composition. as an aqueous solution. Imthisregard, sodium silicate, either added during' milling. or subsequently, such as in aqueous solution, functionsas a binder imparting strength toth coating until it is fired.

In applying the coating composition to the articles of aluminum or aluminum alloys, the metal articles are previously cleaned of grease and other foreign matter, usingsoaps and/or detergents and water, and preferably also with the aid of an alkali or acid bath. The composition in the form of'afiuid suspension or slurry, termed theslip, is then applied to the cleaned metal article as by spraying; brushing, and the, like.

coating-is then fired, preferably after excess surface water has evaporated, to causev the coatingto fuse and to vitrify into a continuous glassy coating. The firing of the coating follows well known procedures, however, as stated, an important feature of the compositions of the present invention is their ability to be fired at temperatures as. low as about 900 F. It will be understood, however, that temperatures above 900? F. may be employed, if desired, although temperatures. much in excess of about 1.000 F. will generally notbe employed for reasons set. for.th previously.

. Two or more co ts of the enamel may be, and Preferably are, applied. This may be. accomplished. by following. the same procedure each time. For instance, after the. first coating has beenapplied and fired as described, a second coating maybe applied and fired in the same manner. In; the event only onecoating is to be. applied, it ispreferable to coat the metal surface with a solutionof. sodium silicate, such as a 5% solution,

prior to the application of the slip. This procedure provides an enamel of good surface texture andadherenca f ree of surface pits which may be associated with the first coating. A top coating o fyanother type or enamel may be applied to a ground or base coat of the enamel of the present .aconsidelfationof the following specific examples which, are-given for the purpose of illustration only and are not. intended to limit the scope of the-inve on in any y- The following materials, all of which wereioi a particle size less than 30 mesh, are mixedinthe indicated amounts:

Partsbyweight um a n e .---.--.--;.-1.-- Sodium fluoride 7.5

Sodium carbonate mqnohydrate 1vf.fi:-:r::=r Bor c ci e uminumme apho B ac rop er Qa sie a.

Sitive; t reduct n: n. x izma atm s here maintain d. i he u ce. Ate i mate tent.- pe ure f ab ut; 18, .0.- h was: com le ely m lts dow to. a flui b u -ame la This talges: about 30-to40 minutes.

Ifhe molten mass is then water quenchedin accordance, with, normalpractice, and the result; ing, frit is dried; The. frit has a O i e; analysis as follows:

Percent-.- by weight The frit is prepared as an enamel by mixing in a porcelain jar mill, per 1 00 parts, by weight, of frit, 2 parts ofsodium silicate, .25 part potassium chloride, and 40- parts of water. This mix;- ture isground in the mill until the residue on a 200 mesh screen is less than 2 grams'per cc. of slip. At this point the slip is'suitable for application as by' spraying, dipping, and the-like.-

The aluminum panel to beenameled'is prepared by Washing with soap and water to remove all traces of grease and other foreign matter. After drying the washed panel itis sprayed withthe slip to provide about 25 ounces of enamel (dry) tothe square-foot. As soon as the surface. water disappears,v the. piece. is: fired at: a temperature of 900 F. for five minutes. Higher temperatures maybe used in. which, case the firing,- time. reduced. For example, at: 9.50 F. the firing time is about 3 minutes,,and1at'1000 E2. the firing time is about 2 minutes.

n ac or c h: p efer ed pract ce a. e ond coat is applied following the same pro.- cedure as used in applying the. first coat. The resultingenamel is free of surface defects. and possesses a high gloss and a. pleasing: blue-green color.

EXAMPLE I mixtur i p par CQDQ EZ Q 0i;

Ear-ts byweight m xtu e is me tedan prep ea ato arm following the procedure of Example I. The resulting frit had the following analysis:

Per cent by weight NazO 11.8

NaF 8.0; A1203 20.3

When ground and prepared as a slip iol1owing the procedure of Example I, the slip is applied to an aluminum panel and fired at 980 1 for 5 minutes. ,Two applications are made. The resulting enamel is a semi-opaque cobalt blue.

A wide variety of ooinpositionmay be prepared following the same general procedure outlined in Example 1'. Thus by way or" further exampies, the following compositions are given in which the proportions of the various oxides in the frit are set forth as percent by weight:

Tablel fi III IV v vi VII viii IX x xi Li1O 3.5 3.5 2.7 3.5 2.4 .95 N220... 13.5 13.1 10.0 14.2 14.0 .57 NaF... 8.2 7.8 7.4 8.1 8.5 .50 A1205-.. 20.7 10.0 18.7 20.0 21.5 .03 B205... 7.0 5.5 5.4 7.0 11.5 .00 P205.... 43.1 41.5 40.0 42.0 35.9 .01 0110... 3.0 3.7 7.0 3.0 4.0 .57

C00..-- 1 oao.-.

Ni T102... 3.7

Considerable modification is possible in the selection of the ingredients, as well as in propor tions thereof, in the preparation of the composition of the present invention without departing from the scope of the present invention.

I claim:

1. A composition in the form of a frit, available for use as a vitreous enamel composition comprismg:

Li20--between about and about mol percent NazO-between about 15 and about 33 mol percent an easily reducible oxide selected from the group consisting of CuO, 000, N10, and Cd0-between about 3 and about 12 mol percent A1203-betWeen about 15 and about 28 mol percent P205-betvveen about and about 32 mol percent and fluorine in an amount equivalent to between about 2 and about 15 mol percent of Na2F2; the sum of the mols of any B203 and of A1202 divided by the mols of P205 providing a ratio between about 0.0 and about 1.6, and the sum of the mols of any B203 and of A1202 and of P205 divided by the sum of the mols of Li20 and of Na20 and of the said easily reducible oxide providing a ratio between about 1 and about 1.5.

2. The product of claim 1 wherein said easily reducible oxide comprises CuO 3. The product of claim 1 wherein said easily reducible oxide comprises C00.

4. The product of claim 1 wherein said easily reducible oxide comprises N10.

5. A composition in the form of a frit, available for use as a vitreous enamel composition comprising:

Li20--between about 8 and about 13 mol percent NazO-between about 25 and about 30 mol per cent an easily reducible oxide selected from the group consisting of CuO, 000, N10, and Colo-between about 1 and about 6 mol percent Al20a-between about 18 and about 20 mol percent ?205-between about 24 and about 28 mol per cent B203-- etween about 2 and about 12 mol percent and fluorine in an amount equivalent to between about 2 and about 15 mol percent of Na2F2; the sum of the mols of 1320s and of A1203 divided by the mols of P205 providing a ratio between about 0.8 and about 1.2, and the sum of the mols of 13203 and of A1203 and of P205 divided by the sum of the mols of L and of N220 and of the said easily reducible oxide providing a ratio between about 1.2 and about 1.4.

6. The product of claim 5 wherein said easily reducible oxide comprises CuO.

Y. The product of claim 5 wherein said easily reducible oxide comprises C00.

'0. The product of claim 5 wherein said easily reducible oxide comprises N10.

9. A composition in the form of a frit, available for use as a vitreous enamel composition comprising:

Li2Obetween about 5 and about 15 mol percent limo-between about 15 and about 33 mol percent an easily reducible oxide selected from the group consisting of CuO, C00, N10, and Cd0-between about 3 and about 12 mol per cent Al2O2between about 15 and about 23 mol perand fluorine in an amount equivalent to between about 2 and about 15 mol percent of Na2F2; the sum of the mols 01" B202 and of A1203 divided by the mols of P205 providing a ratio between about 0.6 and about 1.6, and the sum of the mols of B203 and of A1203 and of P205 divided by the sum of the mols of H20 and of Na2O and of the said easily reducible oxide providing a ratio between about 1 and about 1.5.

10. The product of claim 9 wherein said easily reducible oxide comprises CuO.

11. The product of claim 9 wherein said easily reducible oxide comprises C00.

12. The product of claim 9 wherein said easily reducible oxide comprises NiO.

JOHN W. DONAHEY.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,436,825 Prior Mar. 2, 1948 FOREIGN PATENTS Number Country Date 30,168 Austria 1906 

1. A COMPOSITION IN THE FORM OF A FRIT, AVAILABLE FOR USE AS A VITREOUS ENAMEL COMPOSITION COMPRISING: 